1. Field of the Invention
The present invention relates to a method of fabricating a heat exchanger, and more particularly to a continuous strip which when folded along a pattern imposed thereon, creates a heat exchanger with alternating flow passages.
2. Description of the Prior Art
Heat exchangers provide a means for transferring thermal energy from one fluid stream to another while permitting no mixing of streams to occur. Heat exchangers are often used in indirect evaporative coolers whereby a water spray evaporatively cools scavenger air in the passages thereof.
Early heat exchangers were fabricated by joining separate plates in an adjacent manner so as to provide passageways for air flow. However, the large number of edges needed to be bonded, combined with the ineffectiveness of such bonds, led to problems such as cross leakage and inefficient transfer of energy. Such problems prompted the bonding of the edges of many separate plates to be replaced by the folding of a continuous strip of material.
The continuous strip of material forming most conventional heat exchangers, is usually patterned with fold lines thereon and folded along such lines to arrive at a configuration appropriate for thorough heat exchange. By folding as many edges of the strip as possible, the number of edges that must be bonded is greatly reduced.
Although this process has led to a reduction in the number of edges which require bonding, the pattern on the strip and the manner in which the edges have been bonded has led to limitations on the spacing between walls of the heat exchanger. The spacing of the walls determines the effectiveness of air flow. Spacing is lessened when the parallel walls forming the flow passageways are directly adhered to one another.
An example of such a system is shown in German Publication No. 25 21 351 whereby a heat exchanger is fabricated from a foil-like material folded 180.degree. several times in a zig-zag fashion. An alternating system is achieved, in that the flow through channels are open alternately in directions that are 90.degree. to one another. However, the spacing between the exchanger walls is significantly narrow. This is due to the attachment of the edges of the walls directly to one another. By joining the walls in such a fashion, crossflow is severely inhibited as the air flow passage created by the walls is of an extremely small width.
Ineffective spacing leading to improper crossflow is also a problem with the heat exchanger of U.S. Pat. No. 4,343,355 to Goloff et al. In this system, the unitary material has sections which are fastened through the use of tabs. However, these tabs are welded so as to form a circular wall joining the sections which form the parallel walls of the fluid passages. There are many problems inherent in such a construction. Since a wall of the passageway is formed by a joinder of tabs there is a high probability of air leakage in in the area of the tabs during use. Additionally, in this system an alternating type heat exchanger is not achieved, as air flow is unidirectional.
Another system in which air flow is restricted is in U.S. Pat. No. 4,384,611 to Fung, whereby the end edges of a unitary material are pinched closed along their length to form a wall. Having the walls of the exchanger being formed by such a pinched seam decreases the effectiveness of air flow through the exchanger, as the distance between the parallel walls due to the pinched seam is small. Additionally, the strength and durability of such a pinched seam is speculative, given the tendency of the walls being pinched together to exert forces opposing such an attachment. Also in this system, an alternating pattern of air flow is not achieved.
Another heat exchanger also possessing inadequate spacing between air flow passages is set forth in Japanese Publication No. 53-128047. A unitary strip of material is folded so as to achieve an alternating type heat exchanger. In this exchanger, a tab forms a wall of the exchanger by attachment to the edge of another wall. Although the attachment provides a certain amount of spacing for airflow, a part of the tab itself must be sealed to a parallel wall. Thus, there is a tendency that a portion of the width of the tab will be provided for attachment to a wall, leading to decreased spacing between the parallel walls of the passageway. Additionally, the possibility of air leakage is great with such a construction.